[pulseaudio-discuss] [PATCH v5 0/7] New API for Bluetooth A2DP codecs

Luiz Augusto von Dentz luiz.dentz at gmail.com
Sat Jan 26 15:53:34 UTC 2019

Hi all,

On Sat, Jan 26, 2019 at 3:57 PM Pasi Kärkkäinen <pasik at iki.fi> wrote:
> On Sat, Jan 26, 2019 at 07:39:54AM +0530, Arun Raghavan wrote:
> > On Fri, 25 Jan 2019, at 3:19 PM, Tanu Kaskinen wrote:
> > > On Sat, 2019-01-19 at 18:11 +0100, Pali Rohár wrote:
> > > > This is 5th version of my patch series for modular A2DP codec API and
> > > > aptX support. The only change for v5 is support for switching codecs.
> > > >
> > > > This patch series provides new modular API for Bluetooth A2DP codecs,
> > > > clean up module-bluez5-device and bluez5-util to be codec independent
> > > > and convert SBC codec into this new API for A2DP codecs. Also it adds
> > > > support for aptX, aptX HD and FastStream A2DP codecs.
> > > >
> > > > New codec API is designed in way, that for adding new codec is not
> > > > needed to touch bluez5-util nor module-bluez5-device files. Whole
> > > > codec registration is done in a2dp-codec-util.c file, without need
> > > > to update any header file.
> > > >
> > > > Some A2DP codecs are bidirectional and support backchannel for
> > > > microphone voice. This new A2DP codec API fully supports this feature
> > > > and module-bluez5-device was extended to support microphone voice source
> > > > for codecs which declares such support. FastStream is such codec.
> > > >
> > > > For every A2DP codec is created card profile. When using bluez patches
> > > > from https://marc.info/?l=linux-bluetooth&m=154696260401673&w=2 then
> > > > only those profiles codec profiles are created which are supported
> > > > by remote headset/endpoint.
> > >
> > > As discussed before, I don't think the codec configuration should be
> > > exposed via card profiles. There is need for being able to say "switch
> > > to A2DP" without specifying the codec.
> >
> > I strongly agree with this. Separate profiles for each codec is simply not the way to go -- it's horrible for usability.
> >
> > > It's unclear how priority of the codecs (and their parameter
> > > permutations) should be configured, but it seems quite clear that a
> > > "set codec" operation on a specific card would be useful. If the "set
> > > codec" operation is separate from "set profile" operation, then you no
> > > doubt will ask how to implement the client API for this new operation,
> > > and I don't have a very good answer.
> > >
> > > Georg Chini has been working on a new messaging API, which would be a
> > > good fit for this, but that's stalled (I don't remember if it's waiting
> > > for review or a new version of the patches). If you don't want to be
> > > blocked by that, the alternative is to add new "get bluetooth card
> > > info" and "set bluetooth card a2dp codec" commands to the core protocol
> > > (the card info command would be for enumerating the available codecs),
> > > or to add the commands via a new "protocol extension" similar to what
> > > module-stream-restore, module-device-restore and module-device-manager
> > > do. I would be fine with either approach.
> > >
> > > Adding new commands to the core protocol would be somewhat simpler, but
> > > I'm not sure other maintainers are ok with adding bluetooth specific
> > > functionality to the core protocol.
> >
> > I don't think adding this to the core is necessarily the best option, but I think this is a separate problem.
> >
> > The current patchset should, imo, just take a priority-ordered list of codecs as a modarg and use that (we can choose some default if it is not specified, also ideally based on what codecs are supported on the system -- as I've suggested before, I don't want us to depend on the codec implementation, but I can help deal with that as a separate step).
> >
> > So the modarg approach gives us a static configuration option for people who care about this setting immediately, with a sensible default for most of our users who will not. How we can make this runtime configurable can be figured out separately (for example, with Georg's on-going work).
> >
> Some thoughts about choosing the "best" codec.. which is quite subjective, depending on what the user wants to achieve, for example:
> 1) To get the best audio quality:
> - Example priority of codecs: LDAC (most preferred), HWA LHDC, AptX HD, AptX, AAC, SBC
> 2) To get the smallest/least latency (to lip-sync videos, for gaming, etc):
> - Example priority of codecs: AptX Low Latency (AptX-LL, most preferred), FastStream, SBC
> 3) To get the best battery life on your headset (I've read about some heaphones which have the best battery life with AAC, and for example AptX or LDAC uses more battery on that device)
> - Example priority of codecs: AAC (most preferred), SBC

We could in theory change the codecs on demand but that could
introduce glitches since during the switching the audio cannot be
rendered, because of that Im in favour o user selecting the codec and
then that being remembered by PA when reconnecting. If it is
implemented this way I don't think we need to do much with the
priorities, expect for the very first time we connect, after that it
is up for the user, if he cares, to choose whatever it wants, though
perhaps that is the real issue since some folks here don't want to
reuse profiles to select codecs, even though that already does most of
what we want given that it restores the last used like Ive mentioned

> So yeah, depending on the use case / scenario, the priorify of codecs for negotiation is different.. and the users most probably also want to be able to force usage of specific codec.

Auto setting in not that simple, specially because we can have
multiple client with different type of audio, for instance MP3 and AAC
is only really useful if we can do pass-througth mode but that would
disable other sources since otherwise we would have to encode on the
fly which is expensive with these type of codecs. So Id stick to a
simple ranking of preferred codec for the first connection, which
should probably include only codec which we can encode on the fly, if
the user is not happy with that it has the option to change but that
is about it. We could perhaps think about an option to enable forcing
the codec in case the headset don't honour the last selected, but I
wouldn't enable it by default as it could cause glitches when
reconnecting and attempting to switch.

Most likely only power user would care about these settings, but this
could also be useful for fixing problems with headset that don't
really work properly with certain codec.

> > -- Arun
> -- Pasi

Luiz Augusto von Dentz

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